Hi Uwe,

I'll try to remember to send the scan from home tonight. It is just
under 4 megabytes, so it will take a while to download, but it is ten
pages from the RSGB book, which seems to explain things better than
the ARRL article you have (which is the one I also scanned,
separately). The RSGB pages I scanned includes I think four different
designs, maybe five, as well as some elemental theory.

Yes, I think the Monimatch uses coupled transmission lines, which is
what a microstrip (or stripline) coupler is. The microstrip version
is (generally) a piece of printed circuit board with a ground plane
(uninterrupted copper foil) on one side, and a straight trace on the
other side, which is the "through" line, with another trace parallel
to the first trace and a small distance away, which is the "coupled"
line. You terminate the coupled line at one end (to avoid
reflections), and put a detector at the other end, usually just a
diode detector for SWR monitoring. That tells you the power in one
direction. Then for convenience, you can put an identical coupled
line on the other side of the through-line, and terminate it at the
opposite end compared with the first coupled line, and put a detector
on it at the other end, and that monitors the power in the other
direction. So you get two DC outputs, one for "forward" power and one
for "reverse" power. One important point that is usually
glossed-over, is that diode detectors will respond with an output
voltage proportional to the input RF voltage above some level, but
with an output voltage proportional to the input RF _power_ at lower
levels. You should design the coupling to operate in one or the other
of those regions, if you want to more easily make quantitative sense
of the readings. (An even better way to do it would be to have a
calibrated step attenuator between the "forward" coupled line and the
forward detector, and then adjust the attenuator for equal outputs
from the two diode detectors. Then the attenuator setting tells you
the load's return loss, from which you can find the SWR if you wish.)
It's also possible to use phase-sensitive detectors and get the
complex load impedance...that's essentially what an S-parameter
network analyzer does.

Cheers,
Tom


Uwe Langmesser wrote in message ...
Hi Tom,

No, I don't know the article you mentioned.
I just got the ARRL Antenna Book and there is a plan for a directional
coupler using some plumbing hardware and I could see myself building that,
but again there are some issues about available parts (thru feed caps in
particular).
From my (limited) understanding these couplers would be the aquivalent of
the "plugs" used in meters like the Bird 43 or the URM120.

But I am not sold on this design and would certainly want to look at the
article you mentioned. And yes, I do have a slow phone connection, but if
you are willing I would appreciate if you could send the article as an
attachment.

Now the microstrip coupler you mention, is that what people also call a
monimatch? What are the advantages of one design over another?

regards Uwe



in article , Tom Bruhns at
wrote on 2/8/04 8:47 PM:

This has been a hot topic recently! I just scanned an ARRL article
and a section of the test equipment chapter of the RSGB VHF/UHF
Handbook to send to someone else who is interested in making a
146/440MHz SWR monitor, and I just made a couple 100MHz-6GHz detectors
for someone else who is looking at monitoring SWR at 2.5GHz.

Seems to me the simple way for most folk to do it is to make a
microstrip coupler. You can use surface-mount components for the load
and detector and RF decoupling, and they'll work quite well up into
the GHz region, from my experience. As far as RF decoupling goes, you
should be able to do an adequate job on a circuit board...once the
detector turns the RF to DC, just put shunt capacitance to ground and
series inductance in the line. Pick the inductance as you would for
other VHF work: avoid inductors with self-resonances below the freq
of interest.

You probably have already seen the ARRL article I scanned, but if
you'd like the RSGB one, I could send it. But it's almost 4 megabytes
and may take you a while to download if you have a slow connection.

Cheers,
Tom


Uwe Langmesser wrote in message
...
I have been looking at various designs of VHF SWR bridges, mainly from ARRL
sources like old QSTs and such, and I wonder if anybody here has built a
device like that.
For my experience level some of the old descriptions are just a touch to
cryptic or the design calls for parts which I can't locate (small feed thru
caps are one of those items).
I would love to discuss this with a knowledgable builder.

73 Uwe